The present invention relates to a method of operating an electrostatic precipitator to obtain a pure gas having a predetermined dust content with a minimum energy consumption, wherein the voltage applied to the precipitator is automatically controlled by semipulses.
Published German Application No. 31 14 009* discloses an electrostatic precipitator in which dust is collected under the action of a high d.c. voltage, which is applied between the collecting electrodes and the corona electrodes, and is adapted to be controlled by a thyristor. The precipitator is provided with a control circuit for an intermittent activation of the thyristor in such a manner that the repetition period and/or the pulse width of the high d.c. voltage can be manually or automatically adjusted. The control circuit is intended to improve the collection efficiency of the electrostatic precipitator particularly, in case of high dust resistivities in the range from 10.sup.11 to 10.sup.13 ohm-cm, in which the operation of an electrostatic precipitator is normally unsatisfactory due to the occurrence of reverse corona discharges. FNT *corresponds to U.S. Pat. 4 410 849
In the known control circuit the thyristor is activated in such a manner that the high d.c. voltage is applied during a first interval of time T.sub.1 mounting, e.g., to 0.001 to 1 second, and is interrupted for a second interval of time T.sub.2 amounting, e.g., to 0.01 to 1 second. The ratio of T.sub.1 to (T.sub.1 +T.sub.2), i.e., the ratio of the pulsing time to the pulsing and non-pulsing times, in each switching cycle may be described as the k value and the entire method can be described as "control by semipulses".
A special object of the known method is to avoid reverse corona discharges, which are represented in the current-voltage characteristic by a comparatively very steep rise of the current in conjunction with an only slight voltage rise. This results in a high energy consumption but a low dust collection efficiency in the electrostatic precipitator. However, there is a certain delay between the occurrence of reverse corona discharges and the increase of voltage and/or current effected by the conventional automatic control so that the reverse corona discharges can be substantially avoided and an economical operation of the electrostatic precipitator can be achieved by the use of the semipulses.
It is apparent that the measures proposed in Published German Application No. 31 14 009 are adopted in order to achieve an optimum collection efficiency even when the dust has a high resistivity.
However, the practice of the above-described method does not take into account that operation with an optimum collection efficiency may result in pure gases having quite different dust contents in dependence on the dust resistivity and that these dust contents may be higher or lower than a prescribed value. In other words, the known control is not directed to the actual object of dust collection, namely, to reduce the original dust content from its original value to a value which complies with local emission standards. Whereas it may be desirable from an ecologic aspect to provide a pure gas having a dust content which is much lower than the prescribed limit, that practice will introduce avoidable costs into the production and will tend to reduce competitiveness. A control by means of a system which does not take the dust content of the pure gas into account may achieve an optimum from a technical aspect but may not be economically desirable.